Used as a verb in sewing. To tack means to sew together with long, loose stitches. The purpose of the tacking is to hold two pieces of fabric together and stop them from moving while you use a sewing machine on them, the loose tacking stitches can then be easily unpicked. It is used especially in dressmaking, to hold something together while its tried on, the tacking is easy to undo and redo if the size is wrong.